1. Structure of a CPRI Distributed Base Transceiver Station
In mobile communications system, a radio access network is composed of a BTS (base transceiver station) and a BSC (base station controller) or a RNC (radio network controller), as shown in FIG. 1. Wherein, a distributed BTS is mainly composed of a BBU (base band unit) and a RRU (remote radio unit), as shown in FIG. 2.
In the CPRI protocol, a term REC (radio equipment controller) corresponds to a BBU, and a RE (radio equipment) corresponds to a RRU. The CPRI protocol provides the norm for the interface between a REC and a RE, which belongs to internal interfaces of a BTS and may be connected by way of optical fibers or cables. The norm is available at the web site http://www.cpri.info. The industry currently uses the norm widely to develop a distributed BTS system. The diagram of the interconnection of REC/RE is as shown in FIG. 3.
2. A CPRI Protocol Stack
The CPRI defines the contents of layer 1 and layer 2 of a radio interface. The physical layer uses 8B/10B encoding, and the SERDES line rate supports a series of ranks such as 614.4/1228.8/2457.6/3072/4915.2/6144 Mbps and so on.
The data link layer provides that the period for each basic frame is 1/3.84 M (about 260.42 ns), which is composed of 1 control word plus an IQ area of 15 words, as shown in FIG. 4. Every 256 basic frames compose a super frame, and 150 super frames compose a radio frame of 10 ms, as shown in FIG. 5.
The IQ field of 15 words in a basic frame is partitioned into a plurality of IQ containers (AxC container). The partition result of the 1.2288 g line rate is as shown in FIG. 6. For the WCDMA standard, since its chip rate is equal to the CPRI basic frame frequency, the data of a load sector can be precisely put into an IQ container.
No IQ data mapping methods of standards other than WCDMA is defined in CPRI2.1 and previous versions.
In the latest CPRI4.1 version, three methods are complemented to load a certain standard (such as WIMAX, which may be also applied to CDMA) whose sampling frequency is not integral multiple of 3.84M. They all define the structure of the AxC Container Block, which takes K-chip (K chips) as a period.
Method 1: the period of the AxC Container Block, K=LCM(fs,fc)/fs includs S=LCM(fs,fc)/fc IQ sampling values, and the bit wide of an AXC container (AxC container) differs along with the standards Naxc=2*ceil(M*fs/fc).
Wherein, 1 chip period herein always refers to the chip period of WCDMA, which is equal to the CPRI basic frame period 260.42 ns. The fs in the above equations is a sampling frequency of a standard, fc is the frequency of a CPRI basic frame 3.84M, M is the sampling bit wide of the standard, LCM is the least common multiple, ceil represents rounding up.
Method 2, similar with the method 1, the difference being that the period of AxC-container-block in the method 2 is the frame period of WIMAX 5 ms (larger than that in method 1, being a multiple of the period in method 1).
Method 3, the calculations for the period K for the AxC Container Block and the number S of IQ sampling values included in each period are the same as in the method 1, except that the bit wide of an AXC container Naxc=2*M is equal to one load sector I+Q data volume; Na load sectors may be bundled together and put in Nc containers of one basic frame, Nc=ceil(Na*S/K).
3. Problems Existing in Implementing Multi-Standard IQ Mixed-Mode Transmission Through CPRI
The three IQ mapping methods newly added to CPRI4.1 may implement a specific IQ transmission of a certain standard other than WCDMA. However, for a mixed-mode application scene simultaneously having several standards, as shown in FIG. 7, there exist some problems difficult to solve.
In a multi-standard mixed-mode application, a RRU will support multiple standards, one piece of fiber will simultaneously transmit IQ signals of multiple standards, and a plurality of base band units of different standards will also exist inside an REC, where a switch unit is needed to switch multi-standard IQ signals to all the optical interfaces. For example, a plurality of load sector IQ signals of one base band unit need to be switched respectively to a plurality of optical interfaces and connected respectively to a plurality of REs. Wherein, the high speed interconnection between the switch unit inside the REC and each base band unit may also use a fame structure similar to CPRI.
There will exist many problems in using any of the above three methods when the LCM of a standard's sampling period of the mixed-mode and the CPRI basic frame's period is too large, i.e. K values corresponding to their AxC-container-block are too large (cycle period is K/fc), for example, K corresponding to CDMA2000-1X is 25; WIMAX-10M corresponds to a sampling frequency of 11.2M and a K=12.
Problem 1: if method 1 or method 2 of CPRI4.1 is used, the values corresponding to hit widths of IQ containers (Naxc) of various standards are different from one another because of different sampling frequencies and different sampling bit widths; in order to perform IQ switch of various standards at the REC, the switch unit needs to support simultaneously various different bit widths (difference will be large), which is difficult to implement.
Problem 2: if method 3 of CPRI4.1 is used, in order to perform IQ switching of various standards at the REC, the switched bit widths may be the same if the IQ sampling bit widths of various standards are the same, but the cycle periods of various standards are different. For example, one AxC-container-block of CDMA includes three load sectors IQ, and the cycle period K1=25; one AxC-container-block of LTE1.4M includes two load sectors IQ, and the cycle period K2=2. To support multi-standard IQ mixed-mode switch, a plurality of IQ switch units (one for each standard) may be used inside the REC, which requires a larger expense on logic resources. Or, the plurality of switch units are integrated into one switch unit but the switching is implemented using a common multiple of cycle periods of various standards as the period which however will have a big delay and thus is not acceptable.
Problem 3: if the method 1/2/3 of CPRI4.1 are used, it needs to ensure that the AxC-container-block of each standard is aligned with the boundary of 10 ms radio frame of CPRI (or with an offset of a configurable value) in order to transmit frame timing information of various standards in the situation of mixed-mode RRU cascade. When combining the received signals returned from the downstream, an intermediate-stage RRU firstly needs to analyze the IQ signals returned from the downstream according to the standard, combines with the uplink IQ data of current RRU, and then regroups frames according to the 10 ms frame header Fr from the port TX. Since the cycle period K of each standard is different, each standard needs to process separately, the cost of which is very big. Or, the signals received from the downstream are delayed for a certain length of time to make the boundary of AxC-container-block of each standard aligned with the 10 ms frame header Fr from the direction of the port TX at the current stage, and then the uplink data of the current stage is inserted. However, in this method, the delay of downstream data needs to reach the common multiple of the cycle periods K of all standards, i.e. LCM (K1, K2 . . .) in the most extreme case to make alignment, and the delay is too big to be acceptable.
In addition, the IQ combination, switch and other processes for multi-standard mixed-mode application scene in these schemes are generally related to the standard, and the link's intermediate nodes need to be modified for every newly added standard, which is not flexible.